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Topic: Why is my AAC VBR larger file size than my non VBR AAC (Read 4432 times) previous topic - next topic
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Why is my AAC VBR larger file size than my non VBR AAC

I just used itunes to experiment a bit with different encoding options, and when I encoded the same file once as AAC and once as AAC VBR, in itunes, the VBR one turned out to be slightly larger in file size. Why is this?

I know the VBR in itunes is ABR not true VBR, but why would it be larger?

Why is my AAC VBR larger file size than my non VBR AAC

Reply #1
Any bitrates quoted in relation to VBR, and even ABR, are estimatory only and can be exceeded if the encoder determines that it requires more bits to represent the source waveform.

Not that you provided any information about the two settings! I might assume that you compared CBR 256kbps and ‘VBR 256kbps’ (this latter being a relative misnomer, for the above reason).

Why is my AAC VBR larger file size than my non VBR AAC

Reply #2
hi..
    When a file is encoded in CBR we specify a bit rate..eg 96kps,128kpbs..so while encoding, each frame is encoded only with the specified bits..if the available bits are not enough to encode, then the spectral data is requantized until it fits into the available bits.
But in VBR since there is no bit rate limitation we can encode each frame with maximum bits..(12288 bits for stereo AAC)...this makes the AAC VBR file size larger..
Hope it helps...

 

Why is my AAC VBR larger file size than my non VBR AAC

Reply #3
The exact answer is because the constrained VBR that iTunes uses is actually an ABR with a lower bitrate limit.

Concretely, if you use the 128kbps setting, it uses 128kbps as the minimum bitrate, and then allows for an increase of bitrate when needed.

It cannot be considered VBR, and it's not like the ABR that we're used to.


Note: I don't now if this has changed somewhat with later releases, but this used to be the case.